Signal processing device for automatic-focusing video signal in electronically controlled photographic camera

ABSTRACT

A signal processor for an AF video signal in an electronically controlled camera, comprising: a photosensor (20) of the self-scanning type having an electric charge accumulating part (PD) for accumulating signal electric charge therein which corresponds to the object brightness, an exposure monitor (M1&#39;, M2&#39;, M3&#39;) for accumulating electric charge therein which corresponds to the amount of the signal electric charge accumulated in said electric charge accumulating part, and a dark charge monitor (DM1&#39;) for accumulating dark charge therein, said photosensor being adapted to sequentially transmit the signal electric charge accumulated in said electric charge accumulating part for transfer to an A/D converter (34); and an automatic gain control circuit (37) for amplifying, thereby automatically gain-controlling, the AF video signal produced from said photosensor when said object contrast is low, in such a manner that a low-contrast corresponding level difference, which is the difference between a high-luminance corresponding level and low-luminance corresponding level of an AF video signal arising from a low-contrast object, approaches a high-contrast corresponding level difference, which is the difference between a high-luminance corresponding level and low-luminance corresponding level of an AF video signal arising from a high-contrast object.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement in a signal processingdevice for an automatic-focusing video signal in an electronicallycontrolled photographic camera.

2. Related Prior Art

Electronically controlled photographic cameras have in general anautomatically focusing optical system provided therein for detectingfocuss conditioning of the object to be photographed. FIG. 18 is aschematic diagram showing the arrangement of such automatic-focusingoptical system, wherein 1 denotes a taking lens, 2 the object to bephotographed, 3 a visual field mask, 4 a condenser lens, 5 a diaphragmmask, 6 and 7 a separator lenses forming an image splitting opticalelement, and 8 a photosensor. The automatic-focusing optical systemconsisting of these elements is referenced 9.

In the automatic-focusing optical system 9, the visual field mask 3 ispositioned near a film-equivalent plane 10, which plane 10 is inoptically conjugate positional relationship with the object 2 throughthe taking lens 1; on the film-equivalent plane 10 there is formed asharp image 11 of the object 2 when the taking lens 1 is in suitablyfocused. The condenser lens 4 and the diaphragm mask 5 has the functionof splitting the light passing through the taking lens 1 into twoseparate beams; the separator lenses are situated in a position wihichis optically conjugate with the taking lens 1 through the condenser lens4. The image 11 of the object formed on the film-euivalent plane 10 isreproduced by the separater lenses 6 and 7 as images 11' on two regionsof an electric-charge accumulating part PD of the photosensor 8.

Assuming that the spacing between the suitably focused reproduced images11' is l₀, as shown in FIGS. 19 (a) and 20, the spacing between theimages is narrower than l₀ when the taking lens 1 is positioned to focusbefore the plane 10 as shown in FIG. 19 (b), but the spacing between thereproduced images is wider than l₀ when the taking lens 1 is positionedto focus behind the plane 10. Since the change in the spacing betweenthe reproduced images is proportional to the amount of defocuging of thetaking lens 1, the spacing is measured and the data is processed by anAF-CPU (a central processing unit for automatic-focusing) so as to movethe taking lens 1 depending on the direction and extent of defocusing ofthe lens 1 thereby to position the lens 1 at the suitably focusingposition.

In order to perform such process, it is necesary to process the AF videosignal from the photosensor 8 in a predetermined manner. Electronicallycontrolled cameras have a sinal processor for processing the AF videosignal for performing such predetermined process.

The signal processor for the AF vidio signal comprises: a self-scanningphotosensor having a charge accumulating part for accumulating electricsignal charge therein which corresponds to the luminance of the object,an exposure monitor for accumulating electric charge therein whichcorresponds to the electric signal charge accumulated in said electriccharge accumulating part, and a dark-charge monitor for accumulatingdark electric charge, said self-scanning photosensor being adapted tosequentially transmit the singnal charge, which has been accumulated insaid elctric charge accumulating part, as the AF video signal; and acomparing curcuit for setting a reference level by shifting the outputlevel from the dark-charge monitor in such a manner that the potentialof the electric signal charge accumulated in the charge accumulaing partis driven to a predetermined level, and for providing, upon reversal ofthe reference level and the output level from the exposure monitor, anoutput signal which causes the AF-CPU to provide a command to cause thesignal electric charge accumulated in the electric charge accumulatingpart to be transferred. In this signal processor for the AF videosignal, the transfer command from the AF-CPU is used to drive a drivingcircuit for causing the signal electric charge accumulated in theelectric charge accumulating part to be sequentially transferred as theAF video signal to an analog-to-digital (D/A) converter.

Meanwhile, a dark, low-contrast object will cause a relatively smallamount of signal electric charge to be accumulated in the electriccharge accumulating part, and will thus produce an AF video signal whoselow-contrast corresponding level difference, which is the differencebetween high-luminance corresponding level thereof and the low-luminancecorresponding level thereof, is relatively small. Thus, there is adisadvantage in that the AF-CPU cannot produce proper distance datausing the AF video signal arising from such low-contrast object, sincethe repetition pattern in such AF video signal is weak.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a signalprocessor for an AF video signal in electronically controlled cameras,in which a gain control is performed to magnify to near the appropriateprocess range limits.

A bright, but low-contrast, object will likewise provide a weakrepetition pattern. Thus, it is then preferred that a gain control beperformed for the AF video signal similarly to the case of the darklow-contrast object. Since the level of the AF video signal is generallylowered, the bright but low-contrast AF video signal itself can, whenamplified, exceed the limit process level which is the limit under whicha proper process can be performed.

Another object of the present invention is to provide a signal processorfor an AF video signal in electronically controlled camera, in which,when a bright low-contrast AF video signal is processed, the generallevel of such AF video signal is adjusted to approach the gneral levelof a dark low-contrast AF video signal before it is amplified.

As described above, where processes of amplifying and level-shifting ofthe AF video signal are performed, a changeover from a low-contrastobject to a high-contrast object can possibly result in an AF videosignal which, after the gain control and level-shifting processes,exceeds the appropiate process range, and such AF video signal maysaturate the A/D converter and may thereby be distorted.

A further object of the present invention is to provide a signalprocessor for an AF video signal in electronically controlled cameras,in which, if there occurs a changeover from a low-contrast object to ahigh-contrast object, the gain control is once removed after a levelshifting process of the AF video signal.

Also, an AF video signal has an offset from the process reference levelfor the A/D converter, which offset depends on the circuitcharacteristics. There is a problem caused by the offset in that the AFvideo signal may not appropriately be converted into digital form unlessthe offset is removed before the analog-to-digital conversion.

A still another object of the present invention is to provide a signalprocessor for an AF video signal in electronically controlled cameras,in which the AF video signal is supplied to an A/D converter after theremoval of the offset.

An aspect of the present invention is characterized in that the signalprocessor is provided with an automatic gain control cirtuit forautomatically amplifying the AF video signal from said photosensorthereby to automatically gain-control it when the object contrast islow, so as to bring the low-contrast corresponding level difference,which is the difference between the high-luminance corresponding leveland low-luminance corresponding level of the AF video signal arisingfrom a low-contrast object, near the high-contrast correponding leveldifference, which is the difference between the high-luminancecorresponding level and low-luminance corresponding level of an AF videosignal arising from a high-contrast object.

Another aspect of the present invention is characterized in that thesignal processor is provided with a level shifting circuit which, whenthe general level of the AF video signal corresponding to the objectbrightness has changed, shifts the general level of said AF video signaltoward said process base level, and cooperates with said automatic gaincontrol circuit for adjusting said AF video signal into the appropriateprocess range.

A further aspect of the present invention is characerized in that thesignal processor is provided with an arithmetic processing unit forremoving the automatic gain control, which unit is arranged to onceremove the automatic gain control, when a changeover from a low-contrastobject to a high-contrast object has occured, depending on whether ornot the AF video signal gain-controlled and supplied to the A/Dconverter is within the appropriate process range.

A still further aspect of the present invention is characterized in thatthe signal processor is provided with an offset adjusting circuit forshifting said video signal from said photosensor thereby to adjust theoffset due to circuit characteristics of said video signal from theprocess reference level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing the circuit arrangementof an embodiment, according to the present invention, of the signalprocessor for an AF video signal in an electronically controlled camera;

FIG. 2 is a diagram showing the detail of such circuit;

FIG. 3 is an enlarged view of a photosensor for use in the signalprocessor;

FIG. 4 is an output timing diagram of the AF video signal produced bythe photosensor;

FIG. 5 is a diagram showing the correspondence between the electriccharge accumulating time and the AF video signal output waveform;

FIG. 6 is a diagram showing the AF video signal before beingoffset-adjusted;

FIG. 7 is a diagram showing the AF video signal after theoffset-adjustment;

FIG. 8 is a diagram showing the correspondence between the electriccharge accumulating time and the AF video signal output waveform in thecase of a dark object;

FIG. 9 is a flow chart showing the operation of the AF-CPU in the caseof the dark object;

FIG. 10 is an illustrative diagram showing the gain control in the caseof the dark object;

FIG. 11 is a diagram of the AF video signal in the case of a dark andlow-contrast object;

FIG. 12 is an illustrative diagram of the AF video signal shown in FIG.11, to which a higher gain is given;

FIG. 13 is a diagram showing the AF video signal arising from a brightbut low-contrast object;

FIG. 14 is an illustrative diagram for emplaining the inconveniencewhich can accompany the increase of the gain for the AF video signalshown in FIG. 13;

FIG. 15 is an illustrative diagram showing the AF video signal of FIG.14, wherein its level is generally shifted;

FIG. 16 is a diagram showing the level-shifted AF video signal of FIG.15, wherein a higher gain is given to the signal;

FIG. 17 is an illustrative diagram showing an example of gainchangeover;

FIG. 18 is a schematic diagram showing an automatic focusing opticalsystem; and

FIGS. 19 and 20 are diagrams for explaining the method of detecting thesuitably focused condition using such automatic focusing optical system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the signal processor according to the present inventionfor the AF video signal in an electronically controlled camera willhereinafter be described with reference to the drawings.

FIG. 1 is an overall block diagram of the signal processor, FIG. 2 adetailed circuit diagram of the signal processor, and FIG. 3 a schematicdiagram showing a photosensor of the self-scanning type for use in thesignal processor. In FIG. 3, 20 denotes a photosensor of, for example,the CCD (charge coupled device) type. The photosensor 20 has an electriccharge accumulating part PD for accumulating signal electric chargetherein, a CCD for transfer of the signal electric charge, a transfergate TG, a drain part 21, three exposure monitors M1', M2' and M3' foraccumulating electric charge therein corresponding to the signalelectric charge accumulated in the electric charge accumulating part PD,and a dark charge monitor DM1' for accumulating dark electric chargetherein. The dark charge monitor DM1' is covered with, for example, analuminium film 22.

The photosensor 20 will initiate to accumulate signal electric chargeupon receiving a drive command signal Φ_(int) from a CCD driving circuit20' shown in FIGS. 1 and 2; elctric charge accumulated in the electrccharge accumulating part PD, exposure monitors M1', M2' and M3', and thedark charge monitor DM1' is released into ground through the drain part21 until the drive command signal Φ_(int) is received.

The photosensor 20 initiates to transfer the signal electric chargedepending on drive command signals Φ_(AD), thereby sequentiallytransmitting the AF video signal bit by bit as shwon in FIG. 4. Thedrive command signal Φ_(AD) is outputted upon the output of a transfergate signal Φ_(T). The timing of output of this transfer gate signalΦ_(T) will be described later. The above-described arrangement of theCCD-driven photosensor is already known.

The output signals M1, M2 and M3 from the exposure monitors M1', M2' andM3' are supplied to comparators 23, 24 and 25, respectively. The outputsignal DM1 from the dark charge monitor DM1' is supplied to an input ofa subtractor 27 through a sample hold circuit 26. A reference voltageV1, for example V1=4 volts, is applied to the other input of thesubtractor 27 through a reference voltage setting unit 28, as shown inFIG. 2. The reference voltage V1 setting unit 28 comprises a Zener diodeZD and a feedback circuit (a further circuit) 29 whose one inputreceives a supply voltage of 12 volts through a resistor.

Also, a shift voltage is applied to the one input of the subtractor 27.The shift voltage is produced by a D/A converter 30. The D/A converter30 receives a 6-bit data at its inputs DA5-DA0. This converter providesa shift voltage of a reference voltage V1, for example V1=4 volts, atits output VO when the 6-bit data is "111111", and the shift voltage is3.972 volts when the 6-bit data is "111110". In this way, the outputsignal voltage DM1 can be shifted stepwise at intervals of 28 millivoltsin accordance with the combination of "0" and "1" for the 6-bit data.The voltage of the output signal DM1 is the reference level which willbe described later.

The sample hold circuit 26 comprises a switch 31, a capacitor 32 and afeedback circuit (a further circuit) 33. The switch 31 is closed uponreceiving a dark charge monitor sample hold signal DMS, and the samplehold circuit 26 has the funtion of storing therein the voltage level ofthe output signal DM1 from the dark charge monitor DM1' at the momentsuch signal DMS is received. Said dark charge monitor sample hold signalDMS is produced by the AF-CPU a preset time after the initiation ofaccumulation of the signal electric charge. The reason why the outputsignal DM1 is sample-held will be described later.

The comparators 23, 24 and 25, the subtractor 27 and the D/A converter30, in combination, constitute a comparing circuit which sets thereference level by shifting the output level from the dark chargemonitor DM1' such that the potential of the signal electric chargeaccumulated in the electric charge accumulating part PD can reach apredetermined level, and which, upon reversal of such reference leveland the output levels of the exposure monitors M1', M2' and M3',produces an output signal causing the AF-CPU to produce a transmissioncommand which in turn causes the electric charge accumulating part PD totransmit the signal electric charge stored therein. The function of suchcomparing circuit will be described with reference to FIG. 5.

The AF video signal is supplied to the A/D converter 34 (see FIG. 1) foranalog-to-digital conversion, and is subsequently supplied to theAF-CPU. The A/D converter 34 has an inherent appropriate process rangebounded by a process reference level and a process limit level. An AFvideo signal exceeding these limits cause a trouble in that it willsaturate the A/D converter and will thereby be distorted. In addition,it is impossible to obtain adequete data on the distance to the objectusing an AF video signal arising from a low-contrast object without anyfurther process. It is therefore preferred to use an AF video signalwhose level varies as widely as possible within the appropriate processrange.

Assuming that the surface of the photosensor 20 receives lightuniformly, the exposure monitors M1', M2' and M3', the dark chargemonitor DM1' and the electric charge accumulating part PD initiate toaccumulate electric charge at the moment the drive command signalΦ_(int) is received.

The voltage levels of the output signals from the exposure monitors M1',M2' and M3' will decrease as time passes. Uniquely corresponding tothis, the voltage level of the signal electric charge accumulated in theelectric charge accumulating part PD decreases. The dark charge monitorDM1' receives no light, but dark charge is accumulated as time passes,so that the voltage level ADL of the output signal DM1 also decreases.Here, the voltage level ADL of the output signal DM1 is shifteddownwardly by a predetermined amount. Since the exposure monitors M1',M2' and M3' receives light, the voltage level A of the output signalsM1, M2 and M3 will decrease faster than the reference level ADL' whichis the sifted level of the output signal DM1.

In this embodiment, since the shifted voltage level ADL' is sampled andheld at the moment the dark charge monitor sample hold signal DMS isproduced, the shifted voltage level ADL' is constant after thesample-holding. After a certain period of time, the voltage level A andsuch constant level, labeled ADL", are reversed or transposed together.The reason for the sample-holding of the output signal DM1 is that,without such sample-holding, the reversal in voltage level of thereference level and the output signals M1, M2 and M3 sometimes neveroccurs when dark charge is rapidly accumulated in proportion totemperature rise which causes a rapid decrease of the voltage level ofthe output signal DM1.

Upon reversal of the reference level ADL" and the voltage level A, theoutputs from the comparators 23, 24 and 25 are inverted and the invertedsignals M10, M20 and M30 are supplied to the AF-CPU. Depending on theseinverted signals M1O, M20 and M30, the AF-CPU produces a transfer gatesignal Φ_(t) as shown in FIG. 4. Subsequently, the CCD driving circuit20 produces a drive command signal Φ_(AD) depending on the output of thetransfer gate signal Φ_(t), as shown in FIG. 4, and the AF video signalis produced from the photosensor 20.

The time period since the production of the drive command signal Φ_(int)to the production of the transfer gate signal Φ_(t) is the signalelectric charge accumulating time T during which signal electric chargeis allowed to accumulate in the charge acculating part PD. The chargeaccumulating time T is relatively short when the object is bright, sincethe voltage level of the output signals M1, M2 and M3 will fall rapidly,as shown by reference mark A'. When the object is dark, the voltagelevel of the output signals M1, M2 and M3 will fall slowly and thesignal electric charge accumulating time T is relatively long. Apredetermined AF video signal can be obtained regardless of thebrightness of the object. The waveform shown in the right side of FIG. 5is a schematic representation of such AF video signal. Where the objectis too dark, the transfer gate signal Φ_(t) is sometimes not producedregardless of the length of the charge accumulating time. The AF-CPU istherefore programmed to produce the transfer gate signal Φ_(t) when amaximum charge accumulating time T_(max) has passed, and a relateddescription will be given later.

The AF video signal is supplied to a dark charge sample hold circuit 35and to an automatic gain control circuit 37. The automatic gain controlcircuit 37 is constituted by a comparator 38 and a signal amplifyingpart 39. The dark charge sample hold circuit 35 is constituted by aswitch 40, a capacitor 41 and a feedback circuit (a further circuit) 42,the switch 40 being arranged to be closed upon receiving the dark chargesample hold signal DSH for sample-holding the dark level of the AF videosignal.

The AF video signal is offset-adjusted before it is supplied to theautomatic gain control circuit 37; the AF video signal notoffset-adjusted, before it is supplied to the A/D converter, has anoffset ΔV due to circuit characteristics of the dark charge sample holdcircuit 35 and the gain control circuit 37, as shown in FIG. 6. In orderto remove the offset ΔV, a data, which is supposed to make the darklevel become equal to the process reference level, is beforehandsupplied to a D/A converter 30 through terminals DA0-DA5 by means of theD/A converter 30 and a level shift circuit 43. Further, the AF videosignal is checked to see if its dark level equals the process base level(zero level) before it is supplied to the A/D converter 34.

If the dark level equals the process reference level, the input data isstored in an electronically erasable programmable read only memory(EEPROM) 44. If the dark level does not equal the process referencelevel, then the input data is increased or decreased so that the darklevel can become equal to the process reference level, and that data isstored in the EEPROM 44 when the equality has been reached. Thus, as aresult of such offset adjustment, the AF video signal having the offsetremoved is supplied to the A/D converter 34, and therefore the converter30 and the level shift circuit 43 in combination act as an offsetadjusting circuit which shifts the AF video signal produced from thephotosensor 20 to the process reference level. Therefore, the D/Aconverter 3 is used both for shifting the reference level of thecomparing circuit and for the offset-adjustment.

The level shift circuit 43 includes a shift level varying circuit 44 forstepwise varying the shift level. The shift level varying circuit 44 isconstituted by switches 45 and 46 and resistors, and is capable ofvarying the shift level to 4 steps depending on the combination of thedata applied to terminals DAS1 and DAS2. The function of this shiftlevel varying circuit 44 will be described later, and next thearrangement of the automatic gain control circuit 37 will be explained.

The automatic gain control circuit 37 is constituted by terminals GA4,GA8 and GA16, switches connected to these terminals and resistors, andis adapted to close and open the switches 48, 49 depending on the inputdata applied to the terminals GA4, GA8, GA16 thereby to short-circuitinga selected resistor or resistors, thereby varying the amplificationfactor. The function of this automatic gain control circuit 37 will bedescribed below, with reference to FIGS. 8 through 12.

When the object is dark, then the voltage levels of the output signalsfrom the exposure monitors M1', M2' and M3' will gradually fall but, asshown in FIG. 8, they cannot reach the process limit level within themaximum charge accumulating time T_(max). Under such conditions, if theAF video signal is transmitted without any further process thereto, thenit will have a small low-contrast corresponding level difference whichis the difference between its low-luminance corresponding level andhigh-luminance corresponding level, as shown in FIG. 11. If such AFvideo signal is supplied to the A/D converter 34 without any furtherprocess thereto, then it will provide focus detection data which islittle, causing a relatively large error in the focus detection.

Thus, the AF-CPU will determine whether the maximum charge accumulatingtime T_(max) has elapsed, as shown in FIG. 9 (step S1).

Next, the reference level of the dark charge monitor DM1' is shifted toAGC₀ /2 by the D/A converter 30 (step S2).

Then, the occurrence of the drive command signal Φ_(AD) is waited for(Step S3).

If the output signals M1, M2 and M3 change as shown by a solid line inFIG. 8, the levels of the output signals M1, M2 and M3 will becomereversed with respect to the then reference level AGC₀ /2, and thereforethe transfer gate signal Φ_(t) is produced at the moment the referencelevel is shifted to AGC₀ /2, and the drive command signal Φ_(AD) isthereby produced.

The waveform of the AF video signal then produced from the photosensor20 is typically shown in FIG. 8. This AF video signal is within therange from one half to one times the appropriate process range.

If amplitude of such AF video signal is doubled, then it exceeds thelimits of the appropriate process range. Such too large AF video signalis, when applied to the A/D converter, will cause distortion. Thus, thegain for the reference level of AGC₀ /2 is set at unity.

Next, when the the output signals M1, M2 and M3 change as shown by thebroken line, the drive command signal Φ_(AD) will not be produced evenif the reference level is shifted to AGC₀ /2. Thus, it is determinedwhether or not the drive command signal Φ_(AD) has been produced (stepS4). If the drive command signal Φ_(AD) has not been produced, then thereference level is shifted to AGC₀ /4 (step S5). In this case, the AFvideo signal is between zero and one half times the appropriate processrange.

If the output signals M1, M2 and M3 change as shown by the broken line,then the drive command signal Φ_(AD) is produced. The gain is set at 1Xafter steps S6 and S7 have been performed. The information for settingthe gain at 1X is supplied to the automatic gain control circuit 37through the terminals GA4-GA16.

If the object is still darker, then Φ_(AD) will not be producedregardless of the reference level being set at AGC₀ /4. In such a case,step S8 is performed to cause the transfer gate signal Φ_(t) to becompulsorily produced, and the occurrence of Φ_(AD) signal is waited for(steps S9 and S10).

Such relationship between the shift of the reference level and the gainis shown in FIG. 10. This gain control ensures that substantially equalAF video signals can be obtained regardless of whether the object isbright or dark, and a typical AF video signal shown in FIG. 11 isamplified to a waveform as shown in FIG. 12.

Next, the function of the shift level varying circuit 44 will beexplained, with reference to FIGS. 13 through 16.

In the case of a bright but low-contrast object, the general level ofthe AF video signal is lowered as shown in FIG. 13. If the gain israised for the AF video signal of the generally lowered level, then thelower limit of the AF signal will exceed the process limit level andthus the resultant waveform will be distorted as shown in FIG. 14.

Thus, in the case of a bright but low-contrast object, the shift levelvarying circuit 44 will shift the general level of the AF video signalsuch that it approaches the process reference level (zero level) asshown in FIG. 15, and subsequently the gain is raised and adjusted suchthat the AF video signal is within the appropriate process range asshown in FIG. 16.

Where the general level of the AF video signal is shifted toward theprocess reference level and subsequently the gain is adjusted, the AFvideo signal automatically gain-controlled and supplied to the A/Dconverter 34 may exceed the appropriate process range after a changeoverfrom a low-contrast object to a high-contrast object. In order to avoidthis, a check is performed to see whether the process reference level ofthe A/D converter 34 is over m bits or whether its process limit levelis over n bits, as shown in FIG. 17, and, depending on the result, theautomatic gain control is temporally removed.

Here, an 8-bit A/D converter 34 is used, so that there is a requirementthat m, n≦8.

The above-described embodiment arranged to amplify the AF video signalto obtain an appropriate AF video signal if it arises from a bright butlow-contrast object or from a dark and low-contrast object. Therefore,in the above-described embodiment, if the AF video signal contains noisecomponents before amplification, such noise components are alsoamplified. Since various control actions are made in electronicallycontrolled cameras, such noise components are likely to causemalfunctions of the cameras. It is therefore preferred that the noisecomponents should not be amplified.

In the case of a dark and low-contrast object, the output level from theexposure monitors M1', M2', M3' will fall slowly, so that the chargeaccumulating time, which is defined by the reversal of the output levelfrom the exposure monitors M1', M2', M3' and the reference leveldetermined by shifting the output level from the dark charge monitorDM1', will be longer than that for the case of a bright but low-contrastobject and will approach the maximum charge accumulating time T_(max).Thus, in the case of a dark and low-contrast object, an appropriate AFvideo signal must be derived by amplifying the AF video signal, so thatin this case the contained noise components also must be amplified. Incontrast, in the case of a bright but low-contrast object, the chargeaccumulating time is substantially shorter than the maximum chargeaccumulating time T_(max), so that the charge accumulating time can beextended. Thus, for a bright but low-contrast object, the referencelevel is set by shifting the output level from the dark charge monitorDM1' without amplifying so as to extend the charge accumulating time,whereby an AF video signal can be obtained which has a contrast equal tothat of an AF video signal which can be obtained by amplification. TheAF signal thus obtained by extending the chrge accumulating time willnot contain amplified noise components. For example, for a bright butlow-contrast object represented in FIG. 13, the integrating time isdoubled instead of the gain being doubled. This will provide a signalwhich is equal to the one shown in FIG. 14, but which will contain noisecomponents, if any, the amount of which will only be half as large asthat in the case of an AF video signal obtained by amplification.However, since the high-level corresponding level exceeds the processlimits, distortion will occur. Then, by level shift, an undistorted AFvideo signal shown in FIG. 16 can be obtained, which is substantiallyequal to an AF video signal obtained by a gain of 2× associated withlevel shift.

In order to extend the charge accumulating time in a manner describedabove, firstly, it is determined in Step 1 shown in FIG. 9, whether ornot the maximum charge accumulating time T_(max) has lapsed. If not,then whether or not Φ_(AD) has been generated is determined. If Φ_(AD)has been generated, then the reference level is changed to 2AGC₀ (whichis double the initial reference level) in order to double the chargeaccumulating time, and the occurrence of Φ_(AD) is waited for.

Further, in FIG. 2, reference numerals 50 and 51 denote switches whichare rendered conductive by the application thereto of the referencevoltage, reference numeral 53 denotes a switch which is renderednonconductive by an input thereto through a terminal TESIO, andreference numeral 54 denotes a switch which is rendered conductive bythe input thereto through the terminal TESIO. Further, in FIG. 2,reference marks R1, R2, R3 and R4 denote resistance values of respectiveresistors, and the numerals by the side indicate the multiplicationfactors for the resistance values.

The signal processor for electronically controlled cameras according tothe present invention, having an arrangement as described and shown, hasan advantage in that it allows to process the AF video signal, dependingon the brightness and contrast of the object, into a waveform suitablefor obtaining focus detecting data. In addition, when the signalprocessor is incorporated into elaborate, multifunctional electronicallycontrolled cameras, it will facilitate to design a compact arrangementfor such cameras. Further, since an offset adjustment is made, it allowsto appropriately process the AF video signal.

What is claimed is:
 1. A signal processor for an automatic focusingvideo signal in an electronically controlled camera, comprising:aphotosensor having an electric charge accumulating part for accumulatinga signal electric charge which corresponds to an object brightness, anexposure monitor for accumulating an electric charge which correspondsto the amount of said signal electric charge accumulated in saidelectric charge accumulating part, and a dark charge monitor foraccumulating a dark charge therein, said photosensor being adapted tosequentially transmit said signal electric charge accumulated in saidelectric charge accumulating part; and an automatic gain control circuitfor automatically controlling the gain of said automatic-focusing videosignal produced from said photosensor when said object contrast is low,in such a manner that a low-contrast corresponding level difference,which is the difference between a high-luminance corresponding level anda low-luminance corresponding level of an automatic-focusing videosignal arising from a low-contrast object, approaches a high-contrastcorresponding level difference, which is the difference between ahigh-luminance corresponding level and a low-luminance correspondinglevel of an automatic-focusing video signal arising from a high-contrastobject.
 2. A signal processor for an automatic-focusing video signal inan electronically controlled camera as claimed in claim 1, furthercomprising:an A/D converter having an appropriate process range that isbounded by a process reference level and a process limit level; acomparing circuit for setting a reference level by shifting the outputlevel from said dark charge monitor such that the electric potential ofsaid signal electric charge accumulated in said electric chargeaccumulating part reaches a predetermined level, and for producing anoutput signal to produce a transfer command signal to cause said signalelectric charge to be transmitted for transfer when said reference levelis relatively large as compared to the output level from said exposuremonitor; an offset adjusting circuit for shifting said video signalproduced from said photosensor to said process reference level, so as toadjust the offset of said video signal from said process reference leveldue to circuit characteristics; and a D/A converter for setting theamount of said reference level shift by said comparing circuit, whereinsaid D/A converter is also used for adjusting the offset of said videosignal.
 3. A signal processor for an automatic-focusing video signal inan electronically controlled camera, comprising:a photosensor having anelectric charge accumulating part for accumulating a signal electriccharge which corresponds to an object brightness, an exposure monitorfor accumulating an electric charge which corresponds to the amount ofsaid signal electric charge accumulated in said electric chargeaccumulating part to an A/D converter; an automatic gain control circuitfor automatically controlling the gain of said automatic-focusing videosignal produced from said photosensor when said object contrast is low,in such a manner that a low-contrast corresponding level difference,which is the difference between a high-luminance corresponding level anda low-luminance corresponding level of an automatic-focusing videosignal arising from a low-contrast object, approaches a high-contrastcorresponding level difference, which is the difference between ahigh-luminance corresponding level and a low-luminance correspondinglevel of an automatic-focusing video signal arising from a high-contrastobject; there being an appropriate process range for said A/D converter,said range being bounded by a process reference level and a processlimit level; and a level shift circuit for shifting a general level ofsaid automatic-focusing video signal toward said process reference levelwhen said general level of said automatic-focusing video signalcorresponding to said object brightness has changed, and for cooperatingwith said automatic gain control circuit so as to adjust saidautomatic-focusing video signal within said appropriate process range.4. A signal processor for an automatic-focusing video signal in anelectronically controlled camera as claimed in claim 3, furthercomprising:an A/D converter having an appropriate process range that isbounded by a process reference level and a process limit level; acomparing circuit for setting a reference level by shifting the outputlevel from said dark charge monitor such that the electric potential ofsaid signal electric charge accumulated in said electric chargeaccumulating part reaches a predetermined level, and for producing anoutput signal to produce a transfer command signal to cause saidelectric charge to be transmitted for transfer when said reference levelis relatively large as compared to the output level from said exposuremonitor; an offset adjusting circuit for shifting said video signalproduced from said photosensor to said process reference level, so as toadjust the offset of said video signal from said process reference leveldue to circuit characteristics; and a D/A converter for setting theamount of said reference level shift by said comparing circuit, whereinsaid D/A converter is also used for adjusting the offset of said videosignal.
 5. A signal processor for automatic focusing video signal in anelectronically controlled camera, comprising:a photosensor having anelectric charge accumulating part for accumulating a signal electriccharge which corresponds to an object brightness, an exposure monitorfor accumulating an electric charge which corresponds to the amount ofsaid signal electric charge accumulated in said electric chargeaccumulating part, and a dark charge monitor for accumulating a darkcharge therein, said photosensor being adapted to sequentially transmitsaid signal electric charge accumulated in said electric chargeaccumulating part to an A/D converter; an automatic gain control circuitfor automatically controlling the gain of said automatic-focusing videosignal produced from said photosensor when said object contrast is low,in such a manner that a low-contrast corresponding level difference,which is the difference between a high-luminance corresponding level anda low-luminance corresponding level of an automatic-focusing videosignal arising from a low-contrast object, approaches a high-contrastcorresponding level difference, which is the difference between ahigh-luminance corresponding level and a low luminance correspondinglevel of an automatic-focusing video signal arising from a high-contrastobject; there being an appropriate process range for said A/D converter,said range being bounded by a process reference level and a processlimit level; a level shift circuit for shifting a general level of saidautomatic-focusing video signal toward said process reference level whensaid general level of said automatic-focusing video signal correspondingto said object brightness has changed, and to cooperate with saidautomatic gain control circuit so as to adjust said automatic-focusingvideo signal within said appropriate process range; and an arithmeticprocessing unit for removing said automatic gain control; saidarithmetic processing unit being arranged to remove said automatic gaincontrol after a changeover from a low-contrast object to a high-contrastobject, depending on whether or not said automatic-focusing videosignal, to which said automatic gain control has been applied and whichhas been supplied to said A/D converter, is within said appropriateprocess range.
 6. A signal processor for an automatic-focusing videosignal in an electronically controlled camera as claimed in claim 5,further comprising:an A/D converter having an appropriate process rangethat is bounded by a process reference level and a process limit level;a comparing circuit for setting a reference level by shifting the outputlevel from said dark charge monitor such that the electric potential ofsaid signal electric charge accumulated in said electric chargeaccumulating part reaches a predetermined level, and for producing anoutput signal to produce a transfer command signal to cause said signalelectric charge to be transmitted for transfer when said reference levelis relatively large as compared to the output level from said exposuremonitor; an offset adjusting circuit for shifting said video signalproduced from said photosensor to said process reference level, so as toadjust the offset of said video signal from said process reference leveldue to circuit characteristics; and a D/A converter for setting theamount of said reference level shift by said comparing circuit, whereinsaid D/A converter is also used for adjusting the offset of said videosignal.
 7. A signal processor for an automatic-focusing video signal inan electronically controlled camera, comprising:a photosensor having anelectric charge accumulating part for accumulating a signal electriccharge which corresponds to an object brightness, an exposure monitorfor accumulating an electric charge which corresponds to the amount ofsaid signal electric charge accumulated in said electric chargeaccumulating part, and a dark charge monitor for accumulating a darkcharge therein, said photosensor being adapted to sequentially transmitsaid signal electric charge accumulated in said electric chargeaccumulating part; a comparing circuit for setting a reference level byshifting the output level from said dark charge monitor such that theelectric potential of said signal electric charge accumulated in saidelectric charge accumulating part reaches a predetermined level, and forproducing an output signal to produce a transfer command signal to causesaid signal electric charge to be transmitted for transfer when saidreference level is relatively large as compared to the output level fromsaid exposure monitor; an offset adjusting circuit for shifting saidvideo signal produced from said photosensor to said process referencelevel, so as to adjust the offset of said video signal from said processreference level due to circuit characteristics; and a D/A converter forsetting the amount of said reference level shift by said comparingcircuit, wherein said D/A converter also serves to adjust the offset ofsaid video signal.
 8. A signal process for an automatic-focusing videosignal in an electronically controlled camera as claimed in claim 7,wherein values of said offset are stored in an electronically erasableprogrammable read only memory.
 9. A signal process for anautomatic-focusing video signal in an electronically controlled cameracomprising:a photosensor having an electric charge accumulating part foraccumulating a signal electric charge which corresponds to thebrightness, of an object, an exposure monitor for accumulating anelectric charge therein which corresponds to the amount of said signalelectric charge accumulated in said electric charge accumulating part,and a dark charge monitor for accumulating a dark charge therein, saidphotosensor being adapted to sequentially transmit said signal electriccharge accumulated in said electric charge accumulating part fortransfer to an A/D converter; and a control device for expanding theaccumulating time for said electric charge accumulating part when saidobject has a low contrast.
 10. A signal processor for anautomatic-focusing video signal in an electronically controlled cameraas claimed in claim 9, further comprising:an A/D converter for receivingsaid automatic-focusing video signal; there being an appropriate processrange for said A/D converter, said range being bounded by a processreference level and a process limit level; and a level shift circuit forshifting a general level of said automatic-focusing video signal arisingfrom a bright but low-contrast object, toward said process referencelevel, whereby those automatic-focusing video signals which arise frombright but low-contrast objects can be adjusted within said appropriateprocess range.